Those vehicle steering systems in which the steering unit operated by a vehicle operator and the tires are electrically connected to each other for a synchronized action are known as steer-by-wire systems (which are referred to as SBW systems hereinafter). In a SBW system, a target value of the tire steering angle is computed from the displacement of the steering unit given by a vehicle operator from a steering wheel or a joystick, and an actuator that drives a steering linkage is feedback controlled according to the target value so as to achieve a required tire steering angle.
This feedback control essentially consists of a position control that makes the actual value follow the target value, and it has been customary to include an integral control unit in the feedback loop to minimize the steady-state deviation between the target value and actual value (See Japanese patent laid open publication No. 2001-130430).
Because the steering wheel is not mechanically connected to the tires in a SBW system, the vehicle operator is essentially prevented from directly sensing the road reaction to the tires from the steering wheel. If the road reaction applied to the tires is not transmitted to the steering wheel, the vehicle operator may turn the steering wheel even when the tires are kept immobile, for instance, by abutting a curbstone, without becoming aware of such a circumstance so that the target value of the tire steering angle may become excessive. In such a case, the systems would continue the effort to bring the actual tire steering angle closer to the target tire steering angle, and the actuator load may become so excessive that a mechanical failure could ensue in the worst case. Also, the vehicle operator may feel uncomfortable by the fact that the tires would not change direction even though the steering wheel is turned.
As a measure to alleviate such a problem, it has been proposed to simulate a steering reaction by using an electric motor according to the deviation between the steering wheel steering angle and tire steering angle or the steady-state deviation between the target value and actual value in the feedback control, and transmit this reaction to the vehicle operator via the steering wheel.
However, in a system where an integral control is being performed to the end of minimizing the steady-state deviation between the target value and actual value, when the actual value becomes equal to the target value while the steering wheel is held at a certain steering angle, the steering reaction owing to the existence of a deviation between the target value and actual value disappears, and this deprives the vehicle operator of the expected sense of holding the steering wheel at the fixed steering angle. This is a condition that is never produced in the conventional steering system in which the components are all mechanically connected, and the vehicle operator accustomed to the conventional vehicles may not feel comfortable with such a condition.